Combustion aid for the thermal treatment of products consisting of different materials at baking temperatures and method for the production of the same

ABSTRACT

Commercially obtained combustion aids typically consist of cordierite, fused silica or Si/SiC, which cannot meet the high thermal and also mechanical requirements in the necessary context, or only in a highly limited manner. The invention relates to a glass-ceramic combustion aid in the form of a non-porous keatite glass ceramic without a vitreous surface layer. Said combustion aid has high thermal stability and resistance to thermal shocks, practically no mechanical interaction, especially no porosity and adhesive effects, with the product to be baked, and can also be obtained in large formats having small thicknesses, as a mechanically stable bearing plate.

[0001] The invention relates to a firing aid for the thermal treatment of products consisting of different materials, at firing temperatures of up to 1100° C.

[0002] The invention furthermore relates to the production of such a firing aid.

[0003] In the thermal treatment of products consisting of different materials at firing temperatures, that is, at very high temperatures of up to 1100° C., and in particular in firing ceramics such as bricks, tiles and other ceramic articles, or in converting starting glasses into glass ceramics, support plates are needed for transporting the products to be fired during the thermal treatment; these are known as firing aids. Stringent demands both thermally and mechanically are made of these firing aids. In particular, they must resist the very high firing temperatures, should have sufficient resistance to temperature shock, also known as spalling resistance, and should have only slight interaction with the product to be fired, or in other words should in particular not give up any particles or cause any scratches or adhesion.

[0004] These stringent demands cannot be met, or can be met only with severe limitations, by commercially available material (cordierite, quartz material, Si/SiC).

[0005] The essential limitations are the following:

[0006] cordierite and Si/SiC cannot be furnished in large enough sizes (such as 160 mm×900 mm);

[0007] cordierite is porous and tends to give up particles;

[0008] cordierite and Si/SiC support plates can reduce the impact strength of glass ceramic ceramicized on them very sharply;

[0009] the low spalling resistance of cordierite and Si/SiC, particularly in relatively large sizes, drastically reduce the heating and cooling rates attainable;

[0010] the limited mechanical strength of cordierite and (sintered) quartz material, with large sizes, does not permit thicknesses of less than 12 mm and thus allows only poor energy utilization in firing processes;

[0011] quartz material, at usage temperatures>1000° C., tends to form local cristobalite, which is associated with disruptions (dents, clamshell defects) in the surface smoothness];

[0012] for cordierite, the manufacturer generally guarantees only a limited number of firing operations (<500).

[0013] The object of the invention is to create a firing aid which upon thermal exposure up to temperatures in the range around 1100° C. can be used over a greater number of thermal treatments of the products then in the prior art; that has a high spalling resistance; that exhibits practically no mechanical interactions with the product to be thermally treated and in particular no adhesive effects; and that even in large sizes can be used as support plates with slight thicknesses, even in large sizes.

[0014] With the firing aid for the thermal treatment of products consisting of different materials at firing temperatures as defined at the outset, this object is attained according to the invention in that the firing aid comprises a nonporous glass ceramic, in the form of a keatite glass ceramic without a glasslike surface layer.

[0015] Furnishing a keatite glass ceramic without a glasslike surface layer as a firing aid for the thermal treatment, particularly for firing different materials at temperatures of up to 1100° C., has the following advantages:

[0016] The firing aid

[0017] has no pores and does not give off any particles, or in other words does not leave any “imprints” behind, and makes good impact strength of LAS glass ceramic ceramicized on it possible;

[0018] does not tend to form surface cracks;

[0019] can be used in large sizes (such as 1600×900 mm²) and with small thicknesses (such as 4 mm); that is, even at large sizes with slight thicknesses, it still has an adequate mechanical strength to achieve efficient energy utilization with it. When shipped on rollers in a roller-type furnace, it proves to be advantageous if at least one side of the firing aid, preferably the underside, has a structured and preferably nubby surface;

[0020] has a good spalling resistance (500 to 800° C.) and therefore even at large sizes makes higher heating and cooling rates possible compared to cordierite or Si/SiC;

[0021] in use in roller-type furnaces as well, compared to cordierite it makes more than twice the number of temperature cycles possible;

[0022] even in relatively long-term use, does not exhibit any changes in the surface smoothness from clamshell defects or the development of dents;

[0023] because of its special surface properties, makes a significant reduction in the interaction between the product to be treated and the aid possible. It also proves favorable that the firing aid has a coefficient of thermal expansion of only 1 to 1.3×10⁻⁶/K;

[0024] is temperature stable even at the high firing temperatures. The viscosity of the firing aid, at 1080° C., is greater than 10¹³ dpas.

[0025] The firing aid can be realized with a smoothness on the diagonal of better than 1 mm per 1 m from the ideal smooth surface.

[0026] Because of the high temperature stability, it also makes it possible to fire high-quartz mixed crystal glass ceramic;

[0027] in contrast to cordierite, is partly permeable to IR radiation, which advantageously means that the product to be fired can be radiation-heated from below as well.

[0028] In terms of the method, the object is attained by a method for producing the firing aid having the following steps:

[0029] Furnishing a green glass body in the desired dimensions of the firing aid, and

[0030] Ceramicizing the green glass body with a keatite phase either directly or indirectly from a glass ceramic body, previously converted in the electric furnace from the green glass body, with a high-quartz mixed crystal phase, in a gas or electric furnace.

[0031] In Japanese Patent Disclosure JP 05-339021, a method for smoothing the surface waviness and improving the compaction of articles made of glass is described in which a glass wafer is thermally treated in an annealing furnace on a heat-resistant glass ceramic wafer that is especially smooth. The walls of the furnace likewise comprise a glass ceramic. As the glass ceramic, a glass ceramic with β quartz or β spodumen crystals is described.

[0032] As an advantage to using glass ceramic as a thermal treatment aid, the reference describes the avoidance of abrasion and of the formation of dust or corrosion particles, which could impair the surface of the glass wafer. However, the method described offers no instructions on avoiding the harmful interactions (adhesion) between the glass ceramic firing aid and a product being fired at the desired high temperatures of up to 1100° C.

[0033] The structure described in the reference involves only a discontinuous process. Nor are the nubs on the underside of the firing aid that are advantageous for economical use in a roller-type furnace described.

[0034] The known thermal treatment aid is therefore not usable in roller-type furnaces.

[0035] Setting or adjusting any particular surface condition that prevents the adhesion of the product to be fired resting on it is not possible, nor is there any mention of the possibility of also achieving support plates with a coefficient of thermal expansion of 1×10⁻⁶/K and thus achieving an especially high spalling resistance (up to 800° C.)

[0036] In U.S. Pat. No. 4,187,115, the use of anorthite glass ceramics for such applications as firing aids is mentioned, but not described in detail.

[0037] The keatite glass ceramic support plate of the invention, as a result of special shaping, has nubs on the underside, which avert the usual usage injuries in shipping and handling and as a result protects the interstices, in which peak stresses occur upon loading, resulting in an improved long-term impact strength. In this respect it is especially important that the keatite glass ceramic does not develop surface cracks.

[0038] A suitable peripheral machining of the support plate assures the avoidance of peripheral injuries and long-term strength.

[0039] The aforementioned advantages are more or less apparent depending on the composition of the keatite glass ceramic used for the firing aid of the invention.

[0040] Thus in the temperature usage range given of up to about 1100° C., an LAS glass ceramic that is in the keatite phase proves to be suitable, because in contrast to conventional firing aids for high-temperature use, it has no porosity whatever.

[0041] Surprisingly, glass ceramics as defined by claims 5 and 6 do not exhibit the development of surface cracks, which is usual in other commercially available compositions (such as EUROKERA or NEG material) in a conversion at 1100C to 1200° C. for a period of 1 to 5 hours in keatite. The high conversion temperature is required to enable usage temperatures up to 1100° C. without changes in the keatite glass ceramic in its interior and/or on the surface from long-term temperature stress. The crack development at three conversion temperatures in the keatite glass ceramic material of claim 6 is shown in Table 1 in comparison with EUROKEA (EK) material. TABLE 1 Glass ceramic Claim 6 EK 1060° C./2 h Cracks Yes Yes 1100° C./2 h Cracks No Yes 1140° C./2 h Cracks No Yes

[0042] In the same way, keatite glass ceramic compositions as defined by claim 4 offer the capability of achieving an especially low coefficient of thermal expansion and hence an especially high spalling resistance.

[0043] The composition of such glass ceramics has become known per se from European Patent Disclosure EP 0 220 333.

[0044] For special applications, the good surface properties of the keatite firing aid can also be used in a kind of “sandwich”, by placing keatite plates only a few millimeters thick on other ceramic firing aids.

[0045] Below, the production of a platelike firing aid according to the invention, consisting of an LAS glass ceramic, will be described as an example.

[0046] The LAS glass ceramic is produced as in the prior art by a rolling shaping process with a nubby underside from a glass melt, in sizes up to 1600×900 mm² (or greater) with thicknesses between 3 mm and 8 mm. The plates are cut to size and given a special edge polishing (C shape) to eliminate injuries to the edges. The material is ceramicized either directly in a gas (rolling) furnace with a keatite phase, or it is first ceramicized in an electric (rolling) furnace with a high-quartz mixed crystal phase and then converted into keatite in a gas (rolling) furnace. By means of special tempering, that is, by means of the heating rate in the keatite range, the spalling resistance of the material can be adjusted.

[0047] As already noted, it proves to be a special advantage that the glass ceramic of claims 5 and 6 upon conversion exhibits no surface cracks in the keatite. The conversion to the keatite phase in the gas furnace has the effect that the otherwise glasslike surface layer is avoided, and the firing aid thus has better resistance to adhesive effects.

EXAMPLE OF USE

[0048] LAS plates, on 1600×900 mm² keatite glass ceramic support plates, are converted in an electric roller-type furnace from the glasslike state to the high-quartz mixed crystal phase. The keatite support plates are 8 mm thick and run on rollers that are spaced 300 mm apart. The maximum temperature is approximately 900° C. and the cycle time is approximately 3 hours.

[0049] As described at the outset, the support plates of keatite glass ceramic according to the invention, despite slender dimensions, are mechanically quite stable. 

1. A firing aid for the thermal treatment of products consisting of different materials at firing temperatures, characterized in that the firing aid comprises a nonporous glass ceramic, in the form of a keatite glass ceramic without a glasslike surface layer.
 2. The firing aid of claim 1, embodied as a support plate, characterized in that the underside of the firing aid has a nubby structure.
 3. The firing aid of claim 2, characterized in that the edges of the support plate are rounded and are preferably in the form of a C.
 4. The firing aid of one of claims 1-3, characterized by a composition of the glass ceramic in weight %, referred to the total composition: Li₂O 3.0-5.0 Na₂O   0-1.5 K₂O   0-1.0 Na₂O + K₂O 0.3-2.0 MgO   0-1.5 CaO   0-1.0 SrO   0-2.0 BaO   0-3.0 ZnO   0-2.5 Al₂O₃ 18-27 B₂O₃   0-3.0 SiO₂ 54-70 TiO₂ 1.0-5.0 ZrO₂   0-2.5 TiO₂ + ZrO₂ 3.0-5.5 P₂O₅   0-8.0

and optionally at least one clarifying agent, such as As₂O₃, Sb₂O₃, Sn_(o)2, and optionally with the addition of coloring components, such as compounds of V, Cr, Mn, Fe, Co, Cu, Ni—Se, and Cl.
 5. The firing aid of claim 4, characterized by a composition of the glass ceramic in weight %, referred to the total composition: Li₂O 3.5-4.0 Na₂O 0.4-0.7 K₂O   0-0.2 Na₂O + K₂O 0.4-0.8 MgO 0.2-0.6 CaO <0.1 SrO <0.1 BaO 1.5-2.5 ZnO 1.0-2.0 B₂O₃ <0.5 Al₂O₃  20-23 SiO₂  63-65 TiO₂ 2.0-3.0 ZrO₂ 1.0-2.0 TiO₂ + ZrO₂ 3.5-5.0 P₂O₅   0-0.5

and optionally at least one clarifying agent , such as As₂O₃, Sb₂O₃, SnO₂, and optionally with the addition of coloring components, such as compounds of V, Cr, Mn, Fe, Co, Cu, Ni—Se, and Cl.
 6. The firing aid of claim 4, characterized by a weight in weight %, referred to the total composition: Li₂O 3.6 Na₂O 0.5 MgO 0.5 BaO 2.0 ZnO 1.7 Al₂O₃ 22.0 SiO₂ 64 TiO₂ 2.3 ZrO₂ 1.6 Sb₂O₃ 1.5 V₂O₅ 0.30.


7. The firing aid of claim 4 or 5, characterized in that for the components that are responsible for forming the residual glass phase and for the SiO₂ content, the applicable relationships (in weight %) are: 1.5<Na₂O+K₂O+CaO+SrO+BaO+B₂O₃<5 SiO₂−(Na₂O+K₂O+CaO+SrO+BaO+B₂O₃)<65.
 8. A method for producing a firing aid of one of claims 1-7, having the following steps: Furnishing a green glass body in the desired dimensions of the firing aid, and Ceramicizing the green glass body with a keatite phase either directly or indirectly from a glass ceramic body, previously converted in the electric furnace from the green glass body, with a high-quartz mixed crystal phase, in a gas or electric furnace.
 9. The method of claim 8, in which the surface of the keatite glass ceramic body that receives the product to be fired is removed during operation. 